Four-wheel and all-wheel drive vehicles are popular for their enhanced capabilities in inclement weather and off-highway conditions as compared with two-wheel drive vehicles. Such vehicles necessarily have a more complex drivetrain which, in addition to the primary driveline, employ a secondary driveline, e.g. with additional components, such as a secondary axle and a propshaft, and frequently also a transfer case.
Secondary driveline components introduce additional mass, inertia and friction to the drivetrain, which in turn translates to increased fuel consumption. Although enhanced tractive capabilities are not needed for a vehicle traveling a paved highway in dry weather, all four-wheel and all-wheel drive vehicles permanently carry the additional driveline hardware. In some drivetrain designs secondary driveline components may be arranged whereby they can be selectively disconnected from the primary driveline. The secondary axle road wheels, however, will still be “back-driving” the secondary axle differential through axle-shafts which connect the wheels and the differential. Such back-driving can create parasitic drag that, notwithstanding the secondary driveline disconnect, may nevertheless reduces a vehicle's fuel efficiency.
In an effort to reduce the parasitic drag caused by back-driven secondary driveline components, arrangements for selectively disconnecting a secondary differential from at least one of its respective axle-shafts have been developed. Many of these arrangements further disconnect a secondary axle-shaft from its differential via a dog clutch, i.e. by selectively removing a mechanical interference between an axle-shaft and the differential. Typically a dog clutch is positioned on a secondary axle-shaft such that the dog clutch effectively splits the secondary axle-shaft into a differential-side shaft component and a wheel-side shaft component. Thus positioned, a dog clutch may be actuated to selectively connect or disconnect the two secondary axle-shaft components. FIG. 4 illustrates an example of a prior art differential disconnect utilizing a dog clutch 200 for selectively connecting the two shaft components. As shown, dog clutch 200 couples differential-side shaft component 210A with wheel-side shaft component 210B via an internally splined, axially translatable sleeve 220.
Drive wheels typically generate substantial axial, or thrust loads during a vehicle's operation. In a differential disconnect as described above, it is wheel-side shaft 210B that is primarily subjected to those thrust loads. As a consequence, the wheel-side shaft must be supported on the vehicle with at least one bearing capable of sustaining such thrust loads, such as a thrust bearing 230 shown in FIG. 4. Generally, however, bearings capable of supporting high thrust loads create considerable frictional drag when employed in mounting a rotating shaft. Frictional drag on a wheel-side shaft, even when the shaft is disconnected from its differential, increases parasitic drag on the vehicle driveline and thus increases vehicle fuel consumption.